Flexible pipes of the present type for offshore transportation of fluids are well known in the art. Such pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armoring layers on the outer side of the inner liner (outer armoring layer(s)). An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armor layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114, U.S. Pat. No. 6,085,799 and in Recommended Practice for Flexible Pipe, API, 17B, Fourth edition, July 2008, and in Specification for Unbonded Flexible Pipe, API, 17J, Third edition, July 2008.
The term “unbonded” means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers located outside the inner sealing sheath. These armoring layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
In order to have sufficient strength, in particular to prevent the collapse of the inner sealing sheath, the flexible pipe often comprises an armor layer located inside the space defined by the inner sealing sheath. Such inner armoring layer or layers are normally referred to as a carcass. An unbounded flexible pipe with a carcass is usually referred to as a rough bore pipe. The prior art carcasses are usually made from helically wound and interlocked metal strips, such as for example described in U.S. Pat. No. 6,904,939 and U.S. Pat. No. 6,145,546. However, also the use of profiled elongate elements in a carcass has been suggested in EP 1 475 560 and WO 2008/077409 by the assignee.
However, the carcass in a rough bore pipe often provides an undesirable high resistance to fluid flowing in the bore of the pipe and thereby results in an undesired high loss of pressure along the length of the pipe, in particular when compared with smooth bore, unbonded flexible pipes which do not comprise a carcass.
Furthermore a problem has arisen when using rough bore flexible pipes for producing and exporting gaseous hydrocarbons. This problem has shown to be associated with the flow of gas or fluid containing gas in the rough bore pipe and more specifically with the phenomena of the formation of vortices which occur upon contact with the discontinuities between the turns of the carcass, which results in flow induced pressure pulsations (FIP) from the flexible pipes carcass.
The pulsation induced vibration forces acting on the pipes may lead to damaging vibrations and noise and in particular damage due to vibrations at the ends of the pipe and its connection to stationary structure problems often occur. Such damage may eventually result in leaks.
In U.S. Pat. No. 7,318,454 this problem has been alleviated by providing the rough bore pipe with an anti turbulence sheath which is placed on the inner side of the carcass and which is pierced with holes that are intended to oppose turbulence of the fluid flowing in the pipe.
The object of the invention is to provide a flexible unbonded pipe with a strength comparable to the strength of flexible unbonded pipes of the rough bore type but where the problem related to the undesired high resistance and/or formation of vortices as described above has been reduced in a simple and cost effective way. The inventor of the present invention has found an alternative solution to the problem which has shown to be surprisingly effective and simultaneously be relatively simple and cost effective to provide.